This invention relates to a two-dimensional proportional counter for position sensitive measurement of ionizing radiation in one plane, the counter having a counting chamber with a wire anode grid and at least one wire cathode grid spaced apart from, and above and/or below, the anode grid. The directions of the parallel wires of the grid, or of both grids, used for the readout are disposed preferably orthogonally to one another, and each wire of these two wire grids couples in the pulses generated in them at a defined point of a delay line associated with the particular wire readout grid; from the delay line, the pulses are delivered to an evaluation unit for position determination.
A proportional counter of this type is described, for instance, in a paper by Kaplan et al in the journal NIM (Nuclear Instruments and Methods in Physics Research) 106 (1973), pp. 397-406 and a paper by Gabriel et al in NIM 152 (1978), pp. 191-194. In the proportional counter known from the first of the above papers, three wire grids are retained in separate synthetic resin frames stacked on one another. According to this paper, grid spacings of 3 to 10 mm can thus be attained, with a grid size of 20.times.20 cm.sup.2.
The two delay lines are applied to the cathode wires extending out of the particular frame, which function as readout wires, so that a capacitive coupling of the signals on the cathode wires into the associated delay line is attained.
In the second paper named above (at page 193), the cathode wires are connected directly to individual sections of the delay line; here, the delay line is a so-called lumped delay line, for instance as known from a publication of the CERN LEP Division, "The Delay Wire Chamber Description" by Manarin et al dated Feb. 6, 1985. Here, the delay line comprises coil sections with ferrite cores, with taps attached between the sections and spaced apart from the cathode wires; the signals of the cathode wires are coupled into the delay line at these sections. Capacitors that serve to calibrate the individual coil sections are disposed at the taps; thus, a separate capacitance is associated with each separate coil section.
A proportional counter of this generic type is also disclosed in U.S. Pat. No. 3,772,521 to Perez-Mendez.
Finally, a paper by Bellazzini et al in NIM 190 (1981), pp. 627-638, describes a generic proportional counter in which a continuous delay line, which as in the previously mentioned publications is capacitively coupled to the cathode wires, is used for evaluation. In the proportional counter according to this paper, the distance between the cathode grids and the anode grid is 6 mm, and the counting chamber is assembled from six fiberglass frames, the inner frames of which are correspondingly 6 mm thick and each support one of the three wire grids. The active area of this proportional counter is 25.times.25 cm.sup.2.
To attain high positional resolution, it is desirable on the one hand to dispose the grids used for the readout as close as possible to one another or to the anode grid, in particular to minimize the undesirable parallax dictated by oblique tracks of secondary particles. On the other hand, reducing the spacing between these planes in counting chambers of the aforementioned general size, gives rise to problems relating to mechanical stability and tolerance, which mean that the relative variations in the grid spacings becomes extensive enough to impair the quality of measurement. This is primarily because the distance between the anode grid and the cathode grid, or in other words the readout plane, affects the amplification of the counting pulse, and thus affects its pulse shape and finally the positional resolution. Local tolerance variations within these distances therefore lead to local fluctuations in the charge multiplication and to an impairment of the measurement quality.
Unfortunately, minimizing the spacing between planes, which is desired for the above reasons, also means that because of the shortened mean path of the particles in the counting gas, the primary ionization is low, which leads to a reduction of the pulse amplitudes at the readout grids, yet an increase in the high voltage that could counteract this effect is limited in turn by the aforementioned relative local tolerances in the grid spacings; accordingly, there is an increased tendency of puncture voltage phenomena at points where the grid spacing is less than the mean value. As a result, with very close spacing of the grid planes, only a low pulse height is available. This in turn requires that the electronic means for further processing, that is at least the delay line associated with the readout grids, have an optimal quality, in the sense that the position signal present from one grid should be coupled with as little diminution as possible into the delay line, and should reach the particular end of the delay line with the least possible loss and with as much "shape fidelity" as possible, or in other words without reflection and with the steepest possible rise at the edge; at this end, the position information is then obtained by electronic evaluation, the quality (positional resolution) of this information being determined substantially by how well these conditions are adhered to. A further complicating factor in practice is that the wires of the individual wire planes form capacitors among one another, which affect the signal transmission all the more, the closer the wire spacing.
The previously known systems prove to be unsuitable in this respect: The capacitive coupling in of the signals into the delay line, for example as in the Kaplan et al paper or as in U.S. Pat. No. 3,772,521 to Perez-Mendez, is associated with major signal loss, which necessarily increases the proportion of noise, i.e., lowers the signal-to-noise ratio; the consequence is impairment of the "shape fidelity" of the signal and thus poorer positional resolution. Coupling the signals into a segmental, discrete or lumped delay line as in the Gabriel et al paper requires extremely accurate calibration, which must be performed in each individual case, of the capacitances of each segment, in particular in order to reduce signal reflection that also impairs the "shape fidelity" of the signal and hence the positional resolution. In mass production, this is unattainable at reasonable expense.